1. Technical Field
The field of this invention is crosslinkable fluorocarbon polymers and, in particular, high temperature compositions for wire coatings and the like.
2. Description of Prior Art
Various polymer compositions are known for electrical insulating purposes, such as wire insulation and mold-shaped insulating pieces. However, few compositions are capable of withstanding hostile environments such as those typically encountered in, for example, airplane wiring. In such environments, insulating compositions can encounter mechanical stress, wear, salt-laden moisture, corrosive cleaning fluids, oils and fuels, and low and high temperatures. One of the most important criteria for airplane wire is that it be able to withstand high temperatures without melting when a flash fire occurs, for example.
Some of the existing polymer compositions for hostile environments are polyimide materials, such as Kapton.RTM., an aromatic polyimide material manufactured by the Dupont Company of Wilmington, Del. The polyimide-based wire coatings have good thermal properties, but unfortunately suffer from cracking and embrittlement over time. Modifications which decreased the cracking problem in polyimide insulated wires apparently have lead to excessive stiffness and greater susceptibility to corrosion and chafing. The problem is so serious that a recent article in Defense Electronics, January, 1983, suggests that polyimide wiring harness insulation, especially in exposed areas, has caused short circuits in key aircraft systems.
In another approach to developing durable insulators, efforts have been made to irradiation crosslink so-called "high temperature" fluorocarbon polymers, such as ethylene-tetrafluoroethylene copolymers (ETFE) and ethylene-chlorotrifluoroethylene (E-CTFE) as the insulation. However, conventional radiation crosslinking promoters have not worked well with these fluorocarbon polymers. Because fluorocarbon polymers, such as EFTE and E-CTFE, have high melting points, volatile crosslinking promoters such as triallyl cyanurate and its isomer, triallyl isocyanurate, are ineffective. For a variety of fluorocarbon polymers, temperatures above 250.degree. C. are required for extrusion or injection molding to fabricate shaped articles such as wire insulation, sheets, films, tubing, gaskets and boots. When promoters are added to high temperature fluorocarbon polymers prior to processing, the polymers tend to prematurely crosslink and to form gels or lumps, discolor and often to form voids in the final product.
Various compounds have been proposed as substitutes for conventional crosslinking promoters to form durable, high temperature polymers. See, for example, U.S. Pat. Nos. 3,840,619, 3,894,118 and 3,911,193 issued to Aronoff, which disclose the use of allylic esters of polycarboxylic acids in crosslinking agents for fluorocarbon polymers. See also, U.S. Pat. Nos. 3,970,770, 3,985,716 and 3,995,091 issued to Dhami, which disclose the use of esters of sulfonyl dibenzoic acid as crosslinking agents. Additionally, U.S. Pat. No. 3,894,118 issued to Aronoff discloses crosslinking agents composed of esters of dimethacrylic acid. Despite these numerous disclosures the industry has not been totally satisfied by any of the available crosslinking promoters and many fluorocarbon polymers are still underutilized because they have not responded well to attempts at radiation-induced crosslinking using either the new classes of promoters or the more conventional promoters.
In U.S. Pat. No. 4,353,961 issued to Gotcher, a method is disclosed for forming shaped articles from high temperature fluorocarbon polymers, wherein the polymer is first processed at or above its melting point and then permitted to cool and "imbibe" a promoter before being crosslinked by radiation. This method, which requires immersion of the shaped product in a trough or the like filled with the promoter, poses handling problems and adds a time-consuming, additional step to the manufacturing process.
There exists a need for fluorocarbon polymer compositions suitable for use in high temperature environments and which can be satisfactorily radiation crosslinked in an efficient manner. In particular, there exists a need for fluorocarbon-based compositions, for shaped articles and wire coatings, which can be processed and crosslinked without resort to difficult, time-consuming, post-processing, immersion in promoters.